We have had much of this conversation in another thread on 20" vs 700c.
There are more factors. The advantage to riding in a group is significantly less wind resistance for the pack. Depending on where you are in the pack and the speed of the pack the advantage is between 8 and 15% benefit to the followers. This is even better with a 20" wheeled bike as you can get closer to the rider in front of you. Early on the extra stress you get from riding close in the pack will burn energy too.
On flat ground the main mechanical resistance is the tire road interface. Historically the narrowest tire available with the highest tire pressure will be one of your lowest resistance tires.---There is very little comparative data on rolling resistance in tires. Usually the narrowest tire available for the 406 rim is 20 x 1.1.
I recommend using Strava to train with but do not race it all the time. It is great for keeping track of your workouts on the same route over and over. Built routes in Strava where you expect to ride a lot if there are not a lot of routes already built.
Enjoy the ride